Tilt-rotor aerial vehicles are well known and used both in military (e.g., Bell/Boeing V-22 Osprey) and in civilian applications (Bell Augusta BA-609). As is known to those skilled in the design of such vehicles, they suffer from various deficiencies, such as aeroelastic instability limiting their maximum speed, poor hover efficiency, excessive vibrations, and larger noise levels due to large prop-rotors.
To eliminate or reduce these deficiencies, several approaches have been considered by the prior art. One approach is stopping the rotors in forward flight and propelling the vehicle in such flight by other means, usually by jet engines fixed with the thrust vector substantially parallel to the direction of flight, as have been described in the prior art (see, e.g., US patent documents 2010/0072325, and U.S. Pat. Nos. 5,085,315, 3,592,412, 3,404,852). The aircraft described therein are designed to operate in cruise mode such that the blades of stopped tilt-rotors are folded to minimize the drag.
Folding the blades results in rotor complexity. Thus, another prior art disclosure (U.S. Pat. No. 4,979,698) uses unfolded rotors that are stowed behind the fixed wings and feathered to provide added lift while the aircraft is wing borne. As the tilt-rotors in this case are not powered, they cannot be used for Vertical Take-Off and Landing (VTOL), however, this design is intended to give a high speed jet propelled aircraft only a Short Take-Off and Landing (STOL) capability.
Another patent (U.S. Pat. No. 3,797,783) discloses aircraft with a pair of wing tip mounted tilt-rotors (rotary wings) that are driven for VTOL and tilted, feathered, and stowed in forward flight such that the feathered rotary wings form an operative extension of the fixed wing.
Yet another peculiar group of rotorcraft with some commonality to the disclosed invention are those with forward (canard) tilt-rotors/tilt-wings and stoppable main VTOL rotary wings (see, e.g., U.S. Pat. No. 7,665,688 or WO/2007/014531). The main rotary wings in these designs, however, are not tiltable.
Also well known are Quad Tilt-Rotor (QTR) or quad tilt-wing aircraft (see, e.g., U.S. Design Pat. No. D453,317, U.S. Pat. No. 7,004,426, US Patent Publication No. 2005/0230519, and U.S. Pat. No. 4,982,914), and one design with rotors that are meant to be stopped in flight is disclosed in U.S. Patent Publication No. US 2011/0001020 A1.